Falkvinge: Bitcoin’s Four Hurdles: Part One – Usability I can't make a Bitcoin client, but I could design UX for one.

The current software for Bitcoin downloads and verifies the entire history of the entire Bitcoin network with no limit. Think about what would happen if Paypal or Visa relocated their datacenters into your desktop computer and sent all transactions in the world through your internet connection. Scalable software splits the load so that each computer only handles part of the total work. Bitcoin would need to be modified to be scalable to be able to handle more transactions. The page linked to shows a way to increase performance to roughly the double, but not how to make it scalable.